Transparent layers and compositions are often used in optical applications, wherein light is transmitted through a layer of transparent material for viewing. Several factors are often considered in formulating such transparent layers for specific applications. These factors include for example, optimizing the percent transmission of light through the transparent layer, and increasing the functionality of the transparent layer for altering, e.g. upconverting or polarizing, the light as it passes through the transparent layer. Transparent layers are typically used in various optical applications such as display screens in televisions and smartphones, and light emitting diodes (LEDs), for example. Due to much lower power consumption and longer life compared to other lighting sources, LEDs are increasingly utilized in demanding lighting applications such as automotive headlights and residential lighting. The conversion to using LEDs is due to advances in producing high brightness blue LEDs, which generally emit more lumens per watt than LEDs emitting other colors (e.g. red, orange, yellow, and green).
One disadvantage of using high output blue LEDs as general purpose lighting is that blue LEDs undesirably emit a cold blue light. Therefore, development of blue LEDs has partially focused on converting the blue light to white light having different color temperatures, e.g. cool white light (blueish hue with color temperature ˜10000° K.) to warm white light (yellowish hue with color temperatures ˜3000° K.).
One method of converting blue light from an LED to white light is by transmitting the blue light through a phosphor material capable of emitting yellow light. The conversion from blue to white light proceeds when a portion of the blue light from the LED chip is absorbed by the phosphor material and the absorbed energy excites the phosphor and causes the phosphor to emit yellow light. The yellow light emitted from the phosphor combines with an unabsorbed portion of the blue light transmitted through the phosphor material, to produce white light of varying color tones.
A phosphor material used to produce white light from a blue LED is an aluminum garnet structure, in particular Ce3+ doped Yttrium Aluminum Garnet (YAG) crystals represented by the chemical formula Y3Al5O12. This and other phosphors are being used in LED packages that include an organic silicone polymer encapsulant that surrounds the LED chip. The phosphor is included in the form of a disc covering the organic silicone, or is dispersed into a silicone polymer matrix and formed into a composite dome or encapsulant for the LED chip.
However, the organic silicone used as the matrix material for the LED package tends to degrade over time from exposure to light and heat produced by the LED chip. Such degradation of the silicone results in undesirable discoloration of the silicone and reduces the output, and hence the useful lifetime, of the LED package.
In the case of Ce3+ doped YAG phosphor, which are dispersed in the silicone dome/encapsulant, blue LED chips emit light at ˜460 nm wavelength. This light goes through the silicone-phosphor material. The phosphor absorbs part of this blue light and due to fluorescence, emits yellow light in a broad band centered around 550 nm. The blue light (˜460 nm) transmitted through the silicone-phosphor material is mixed with the yellow light (˜550 nm) emitted by the phosphor, and thereby produces white light. In general this white light has an undesirable cool color temperature.
In this regard, the white light emitted by the LED package has an undesirable cool color temperature (i.e. blueish), instead of a desired warm color temperature (i.e. yellowish) similar to traditional incandescent light bulbs. Further, organic silicone used as a polymer matrix degrades during the lifetime of the LED, causing a shift in color shade and/or output of the LED package and thereby decreases the useful lifetime of the LED package. Therefore an improvement in the technology is needed.
In order to produce warm white light, additional red shift is needed from the phosphor. For this, various phosphor technologies, such as mixing of different yellow and red phosphors, and phosphors based on host crystals other than YAG, such as La—AG, Gd—AG, Lu—AG, nitrides and oxynitrides, oxides, oxyhalides and halides are being pursued with different activators such as Ce3+, Eu2+, Yb2+ so on.
Although satisfactory in certain respects, a need remains for an improved white light emitting LED package and phosphor-containing layers in other applications. In particular, it would be desirable to ensure that the matrix material in which the phosphor is dispersed has improved resistance to degradation, such as yellowing or causing a shift in color shade or output.